Add methods for logging with level as argument

[arxeiss]

Adding generic Log, Logf, Logw and Logln methods to Sugared logger.

When I need to do a wrapper around Zap logger to pass that into 3rd party library for unifying logging, it might come handy to use zap.Log(level, ...) instead of switch or if-else if chain.

However, now I need to do a same wrapper but with Sugared logger. And that doesn't support general Log method.

More description in #1405

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (70f61bb) 98.42% compared to head (24ebbfd) 98.42%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1406   +/-   ##
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  Coverage   98.42%   98.42%           
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  Files          53       53           
  Lines        3495     3503    +8     
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+ Hits         3440     3448    +8     
  Misses         46       46           
  Partials        9        9           

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